1. Technical Field
The invention relates generally to the field of data compression, and more specifically, to compression of hyperspectral or multispectral image data.
2. Related Technology
Recent advances in satellite and aerial imagery systems have made it possible to collect voluminous amounts of satellite and aerial imagery data. The sensors used in generating the images are typically characterized as either “multispectral” or “hyperspectral”. Multispectral sensors collect images of a terrain or landscape and provide a handful of wide spectral bands of imagery. These bands encompass the visible, short wave infrared, and, in some cases, thermal infrared portion of the electromagnetic spectrum.
In recent years, there has been increased interest in the field of hyperspectral remote sensing. Hyperspectral imagers are a new generation of remote-sensing instruments that record the sensed optical energy in a number of narrow contiguous wavelength channels. They can collect image data in hundreds of spectral channels simultaneously and over wavelengths ranging from ultraviolet to thermal infrared. The spectral measurements contain information from reflected, or emitted, energy from a surface and the effects of the intervening atmosphere. Hyperspectral data from remote sensors is used in a variety of applications including geology, oceanography, agriculture, ecology, medical imagery, and atmospheric science.
The large number of bands in hyperspectral systems leads to a sharp increase in data volume compared to multispectral systems such as LandSat. As a consequence, compression of hyperspectral data to facilitate real time transmission and/or subsequent storage has become an important research endeavor. Because one of the primary purposes of using hyperspectral sensors is to identify features, such as buildings, crops, or identify minerals, by their spectral signature, maintaining the spectral integrity of each pixel is very important. The spatial quality of the individual band images must also be preserved. The task of compressing hyperspectral data is essentially an optimization problem, balancing image quality and spectral integrity against data compression ratios and processing requirements.
Lossy compression algorithms for hyperspectral and multispectral images can be roughly categorized by how they exploit redundancies in the spatial and spectral dimensions. The first group of algorithms is comprised of single stage methods that do not differentiate between the spatial and spectral directions. Examples in this group include various Vector Quantization (VQ) schemes to compress hyperspectral data. Other non-VQ systems have also been recently introduced. Examples include the use of bijection mappings onto zero-trees. The 3D wavelet transform is another single stage method. For example, JPEG2000 privates several options to compress 3D data cube.
The second group of approaches for lossy multichannel image compression relies on two stages of data processing that exploit the spectral and spatial redundancies separately. Typical approaches are to use the Principal Component Analysis (PCA), Irreversible Component Transformation (ICA), Linear Mixing Model (LMM), or wavelets to spectrally decorrelate the hyperspectral data, followed by an adaptive discrete cosine transform (DCT) or discrete wavelet transform (DWT) coding technique to compress along the spatial directions.
A method for compressing hyperspectral data is disclosed in U.S. Pat. No. 6,167,156 to J. A. Antoniades et al. and is discussed in “Bowles, J., Chen, W., and Gillis, D., “ORASIS framework—benefits to working within the linear mixing model”, IEEE 2003, pp. 96-98.